Several kinds of processes for microorganism cultivation have been developed for different applications. These processes may be conveniently classified according to the chosen operation mode: batch, fed-batch, continuous, semi-continuous (repeated batch), and their variations. In a batch operation, neither substrate is added after the initial charge nor the product is removed until the end of the process. Some phar­maceutical products are produced by this process, but generally, batch operation is not commercially attractive [50], where its main application is in food and beverage production. Conventional batch process can lead to inhibitory concentrations of substrate in reactors or even to formation of undesired products through direct metabolic pathways of the organism, decreasing the yield and/or productivity of the system.

The fed-batch process has been used in the cultivation of baker’s yeast since 1920 [47]. However, Yoshida et al. [117] were the first to use the term “fed-batch process” in cataloging. Whereas the whole substrate is added at the beginning of cultivation in a batch process, in the fed-batch process one or more nutrients are added to the fermentor during cell growth, while cells and products remain in the fermentor until the end of operation [14].

The fed-batch process does not require any special piece of equipment in addi­tion to equipment required for batch fermentation. It is only characterized by a little longer overall time cycle that is certainly acceptable by industrial practice where at present very effective procedures for sterilization and preventing contamination are commonly utilized [59] .

The better operating procedure for this system is to start with small amounts of biomass and substrate, and to add more substrate when the greatest part of the initial substrate is already consumed by the microorganism [42] . The inlet substrate feed should be as concentrated as possible to minimize dilution and to avoid process limitation caused by the reactor size. According to Lee et al. [56], the feed control strategies are: simple indirect feed-back (single-loop) methods; nutrient feeding according to inferred substrate concentration or specific growth rate; and predeter­mined feeding strategies.

Two cases of fed-batch process can be considered: the production of a growth — associated product and the production of a non-growth-associated product. In the first case, it is desirable to extend the growth phase as much as possible, minimizing the changes in the fermentor as well as the specific growth rate, production of the product of interest, and avoiding the formation of by-products. For non-growth- associated products, the fed-batch process would be carried out in two phases: a growth phase in which cells grow up to the required concentration and then a pro­duction phase in which carbon source and other requirements for production are fed to the fermentor [59] .

In the fed-batch cultivation, volume variation may happen depending on the sub­strate concentration added and the rate of evaporation in the system [14]. In the fixed volume fed-batch process, the limiting substrate is fed without diluting the culture. The culture volume can also be maintained practically constant by feeding the growth limiting substrate using a very concentrated solution. Alternatively, the substrate can be added by dialysis or, in a photosynthetic culture, radiation can be the growth limiting factor without affecting the culture volume [32] . Contrarily, the variable volume fed-batch process is one in which the volume changes throughout the cultivation time due to the diluted substrate feeding in the system.

Fed-batch culture has been widely employed for the production of various bio­products including primary and secondary metabolites, proteins, and other biopoly­mers. It is used aiming to overcome different difficulties in cell cultivation. When a nutrient with high viscosity is used, this process usually reduces the viscosity of the medium, and toxic effects of components can also be limited by dilution [113].

The advantages of this process include: deviation of cell metabolism via the formation of the desired product; prevention of catabolite repression; prevention of formation of toxic substances in microbial metabolism; and control of the specific growth rate [116]; the main advantages are the possibilities of controlling both reaction rate and metabolic reactions by substrate feeding rate [36].

The fed-batch process has been also used to prevent or reduce substrate-associ­ated growth inhibition by controlling nutrient supply. Since both overfeeding and underfeeding of nutrient is detrimental to cell growth and/or product formation, the development of a suitable feeding strategy is critical in fed-batch cultivation. Fed — batch fermentations can be the best option for some systems in which the nutrients or any other substrates are only sparingly soluble or are too toxic to add the whole requirement for a batch process at the start. This process is particularly important in A. platensis cultivation. In the next items, the factors that affect A. platensis growth are discussed, aiming to correlate them with the employment of the fed-batch process.